Author: Xiangli Cui, Wanli Huo, Jie Li, Lingling Liu, Tao Ma, Yuanyuan Wang 👨🔬
Affiliation: Institute of Health and Medical Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Hefei Ion Medical Center, Hefei Cancer Hospital, Chinese Academy of Sciences, the Zhejiang-New Zealand Joint Vision-Based Intelligent Metrology Laboratory, College of Information Engineering, China Jiliang University 🌍
Purpose: In practice, treatment planning systems (TPS) of proton therapy require a CT calibration curve, which relates Hounsfield units (HU) to relative stopping power (RSP). Single-energy CT as a standard imaging modality in proton therapy, stoichiometric calibration is considered to be the most accurate to build HU-RSP conversion.
Methods: The tissue-substitutes with known elemental composition (CIRS 062MA) was scanned with single-energy CT. With MIM system, the HU of each tissue-substitute could be measured. Simultaneously, with the measured HU and elemental composition of each tissue-substitute, a stoichiometric fit was performed. And then, the CT number and RSP were calculated for 68 human tissues, which were from ICRP23, Woodward and ICRU44. The CT calibration curve can be obtained by a piecewise linear fitting to human tissues. Finally, the RSP values of tissue-substitutes and three animal tissues were measured with proton therapy treatment system and zebra to validate the CT calibration curve.
Results: For 140kV, the relative differences of ranges between the values calculated with TPS and values measured with zebra were larger for bone tissue-substitutes than others. For soft tissue and lung tissue, the relative differences of ranges were less than 2%. For animal tissues (fat, liver, muscle) tissues, the relative differences of ranges were -0.6%, -1.2% and -0.18%.
Conclusion: the relative differences of ranges for tissue-substitutes and animal tissues have proved that the CT calibration curve built with the stoichiometric method could be used in TPS. However, there is a bigger deviation for the bone tissue, which needs to improve it.